In accordance with an increase in demand for an image display device such as a LCD, a PDP, an organic EL display, a SED, and a CRT display, demand for an optical film as a main component of the image display device has been rapidly increasing for the purpose of obtaining high-quality image thereof. The optical film is a film provided with various optical functions for achieving prevention of reflection of external light, enlargement of viewing angle, correction of optical unevenness, and the like.
The optical film is generally a multilayer film having a multilayer structure composed of a base material as a base, and an upper layer deposited thereon. The base material is typically a transparent film including a polymer as its main component. Among the base materials, the demand for a base material formed of polyester has been increasing, because the base material has features such as excellence in transparency, dimensional stability, chemical resistance, low hygroscopicity, and the like. The upper layer includes a polymer as its main component and additives for providing various optical functions for preventing of reflection of light on the optical multilayer film and the like. As the upper layer there are an antireflection layer, a prism layer, a light scattering layer, and the like, for example. By arbitrarily deciding combination between the base material and the upper layer, it is possible to readily form various optical films such as a prism film, an antireflection film, and a light scattering film for use in the LCD, and further an infrared ray (IR) absorption film, an electromagnetic wave shielding film, a toning film, an antireflection film, an antiglare film, a hard coat film, and the like for use in the PDP, for example.
However, in a case where adhesive strength between the base material and the upper layer is low, the upper layer may be peeled from the base material, thus causing light leakage and making it impossible to prevent light from reflecting. Therefore, the adhesive strength is important in the optical multilayer film, however, it is difficult to enhance the adhesive strength up to a predetermined level since the adhesive strength tends to be easily affected by material composition of the base material and the upper layer, irregularities on the contact surface, formation condition of each layer, and the like. In view of the above, for example, in Japanese Patent Laid-Open Publication No. 2001-294826, there is disclosed an optical multilayer film in which the base material contains polyester, and an adhesion layer including polyester formed thereon, thus enhancing the adhesive strength.
Moreover, since the optical multilayer film is composed of a plurality of materials, whose refractive indices are different from each other, such as the base material, the adhesion layer, and the upper layer, light is easily reflected on the interfaces therebetween. Further, when light is reflected on the interfaces, the reflected light interfere with each other to cause a phenomenon in which rainbow color is appeared (rainbow unevenness). Accordingly, displaying quality in using the optical multilayer film drastically deteriorates. At present, a refractive index of a typical base material formed of polyester is approximately 1.65 that is relatively high. Accordingly, there is proposed a method in which the refractive index of a layer next to the base material is increased so as to decrease the difference in refractive indices between the base material and the layer next to the base material. For example, in Japanese Patent Laid-Open Publication No. 2004-054161, there is disclosed a multilayer film including an adhesion layer containing fine particles as a predetermined metal oxide so as to have higher refractive index. In Japanese Patent Laid-Open Publication No. 2005-097571, there is disclosed a multilayer film including a layer in which coating liquid containing a water-soluble titanium chelate compound, aqueous polyester, and water is applied to a base material formed of polyester, and a hard coat layer. Furthermore, in Japanese Patent Laid-Open Publication No. 2000-111706, there is disclosed a multilayer film including a base material, an adhesion layer and an upper layer in which the differences in refractive index between the base material and the adhesion layer and between the base material the upper layer are adjusted by focusing on refractive index of polymer and arbitrarily making a decision.
The rainbow unevenness also occurs by thickness unevenness of each layer. In particular, in a case where thickness unevenness occurs on the upper layer, the reflected light becomes more intense at a certain thickness, and rainbow unevenness is more apparent on the multilayer film, thus causing a problem. In view of the above, in Japanese Patent Laid-Open Publication No. 2003-177209, for example, there is disclosed a method in which a film is produced while adjusting a refractive index of the adhesion layer and the film thickness thereof so as to prevent occurrence of rainbow unevenness. Further, in Japanese Patent Laid-Open Publication No. 2005-178173, there is disclosed a multilayer film including a primer layer having higher refractive index by adding inorganic fine particles having titanium dioxide as its main component to at least one of surfaces of a transparent base material.
In any case describe above, for the purpose of preventing rainbow unevenness, fine particles, a chelate compound, or the like is added to the adhesion layer. However, in this case, the fine particles, the chelate compound, or the like precipitates between the base material and the adhesion layer, and between the adhesion layer and the upper layer, and then the adhesive strength therebetween decreases, thus causing a problem. Furthermore, when a large amount of fine particles are added for the purpose of increasing the refractive index, strength of each of the layers decreases, thus consequently causing decrease in strength of the film as a whole. Additionally, in Japanese Patent Laid-Open Publication No. 2003-177209, there is disclosed a method in which polymer having a desired refractive index is arbitrarily selected to be used and thereby a refractive index of an adhesion layer is adjusted. However, such a polymer is expensive mostly, and therefore manufacturing cost increases, thus causing a problem.